Tooth bleaching typically involves the application of a peroxide containing composition to the surface of the tooth enamel to achieve a desired whitening effect. The most common peroxide donor ingredients are, but are not limited to, carbamide (urea) peroxide and hydrogen peroxide. The latter has become the tooth bleaching material of choice due to its ability to whiten teeth faster than higher concentrations of carbamide peroxide. Additionally, aqueous tooth whitening gels have proven desirable due to the hydrating effects on the structure of the tooth, reducing the likelihood of tooth sensitivity. As such, it is desirable to achieve stable aqueous hydrogen peroxide tooth whitening gels for commercial preparation.
Aqueous hydrogen peroxide tooth bleaching formulations have their limitations however. Until recent years, stable aqueous hydrogen peroxide tooth bleaching gels have been virtually non-existent. Hydrogen peroxide is a powerful oxidizing agent and an unstable compound that decomposes readily over time into water and oxygen. Certain chemical and physical influences can accelerate the rate of decomposition and need to be controlled for a stable tooth whitening gel to exist. Temperature, pH and errant metal ions all have a profound effect on the decomposition of hydrogen peroxide, particularly in an aqueous formula. Aqueous tooth whitening gels comprising high levels of hydrogen peroxide are defined as comprising an available hydrogen peroxide concentration equal to or greater than about 15% of the total composition weight up to and including about 40% of the total composition weight. Generally, these high level hydrogen peroxide compositions require the gel to be refrigerated to reduce the rate of decomposition, thus enabling an increased shelf life. This process is difficult as well as expensive for commercial distributors to maintain adequate shelf life for salable product.
To combat these limitations, high level hydrogen peroxide bleaching compositions are generally delivered to the end user in a two-part system packaged form that maximizes peroxide stability. Packaging stabilized hydrogen peroxide solutions separately from other formulation ingredients is one method that enables manufacturers to meet required shelf life parameters. Two types of separation methods are dual barrel syringes and liquid hydrogen peroxide/powder systems. The disadvantages to these types of packaging are that the hydrogen peroxide must be mixed at the time of placement in the dental operatory, thus resulting in a loss of valuable chair time and the unwillingness for patients to prolong the tooth whitening experience. Additionally, due to hydrogen peroxide's highly corrosive properties, peroxide burns from the splatter resulting from the spatulation of the mix process are always a concern. There also exists the potential for a non-homogeneous mix
In office bleaching by dentists is the primary end use for high level hydrogen peroxide tooth whitening compositions. Therefore, it is imperative that the whitening compositions be formulated as one-part (no-mix) systems and maintain adequate shelf stability in order to guarantee efficacy at the time of placement by the dentist within the patient.
Most distributors of in-office dental products require that a suitable shelf life be maintained for any given product. Further, most distributors rely on a product to maintain a two year shelf stability. Shelf stability is generally determined amount of time to ensure a abnormal or exaggerated storage conditions for a predetermined amount of time to ensure a product's stability under normal storage conditions. Room temperature storage conditions, for a single component tooth bleaching composition, are desirable in order to eliminate costly and inconvenient storage problems. To a greater degree, a two year room temperature storage guarantee is what is required for distribution of a single component aqueous tooth bleaching composition.
The pH of an aqueous hydrogen peroxide tooth whitening composition also has great bearing on the stability of the gel. Hydrogen peroxide solutions are strongly acidic and maintain their stability in acidic pH formulas. Stable aqueous hydrogen peroxide tooth whitening gels can be formulated in the acid pH range. However, bleaching compositions in the acidic pH range (pH 2.0-5.5) are prone to the demineralization of dental enamel by the solublizing of calcium ions from the tooth surface. This reduction in surface enamel leads to tooth sensitivity and discomfort for the patient.
Additionally, it has been shown that peroxide containing tooth bleaching compositions are more effective at whitening teeth at higher pH levels than at lower pH level. Therefore it is desirous to obtain a single component, aqueous tooth bleaching composition with high levels of hydrogen peroxide that can maintain a non-refrigerated, two year shelf stability while exhibiting a neutral or basic pH or a pH of greater than about 5.5. Unfortunately, to date, no such commercial formula exists.
Recent technology has shown single component, high-level aqueous hydrogen peroxide gels to be stable for short periods of time at room temperature. It is known that the presence of stabilizing agents within the aqueous hydrogen peroxide tooth whitening formula can extend the life of the formula by slowing or eliminating the rate of peroxide decomposition. Decomposition of the peroxide within the formula can result in viscosity loss of the composition, expansion of the composition due to the evolution of oxygen gas and/or the loss of available hydrogen peroxide content.
U.S. Pat. No. 6,555,020 to Chadwick et al disclose stable aqueous tooth whitening compositions containing high levels of hydrogen peroxide. Chadwick teaches the utilization of aminocarboxylic acid/salt stabilizing agents to prolong room temperature shelf stability and to maintain the gel characteristics of the composition. The stabilizing agents are selected from a group consisting of CaNa2EDTA, Na4EDTA, and CDTA. However, Chadwick discloses that the greatest shelf stability is for a term of 84 under room temperature storage conditions. While 84 days (12 weeks) is a significant stability period for an elevated hydrogen peroxide level tooth whitening formula, it fails to show long-term stability under exaggerated conditions and can only be translated into a 12 week stability period for the composition. Thus, it falls considerably short of a desired two year shelf life required to become a salable product through commercial distribution.
U.S. Pat. No. 5,858,332 to Jensen et al. teach of a stable, one-part, premixed hydrogen peroxide tooth whitening composition that utilizes a bleaching agent stabilizer that ties up errant metal ions that can result in decomposition of the bleaching agent within the tooth whitening composition. Jensen et al. describes a bleaching agent selected from a group consisting of aqueous hydrogen peroxide, carbamide peroxide, sodium perborate and mixtures thereof. Additionally, the whitening composition comprises a single thickenening agent, a carrier, and a bleaching agent stabilizer selected from a group of edetate disodium, EDTA, oxine EDTA, calcium disodium EDTA, adipic acid, succinic acid, citric acid, tin nitrates, tin phosphates and mixtures of the foregoing.
Jensen et al. claim a stable one-part dental bleaching agent that maintains at least 95% of its original strength 1 month after its manufacture and at least 60% approximately 3 months after the compositions manufacture. However, Jensen et al. makes no mention of the stability under exaggerated and/or accelerated storage conditions and it is understood that the stability occurs under room temperature conditions and for a maximum of 60-95% available hydrogen peroxide content after 3 months storage time.
U.S. Pat. Nos. 6,488,914; 6,331,292 and 6,221,341 to Montgomery detail aqueous tooth whitening compositions with neutral or alkaline pH levels. Montgomery teaches one-part, aqueous tooth whitening compositions that are formulated to be stable at high pH levels (6-10). The significance of this teaching is the ability to detail an aqueous hydrogen peroxide tooth whitening composition that is stable under neutral or alkaline conditions with a high water content within the composition (>70%). However, Montgomery claims an available hydrogen peroxide content of less than 15% of the total weight of the composition, which falls far short of an elevated or high hydrogen peroxide concentration within the composition. Additionally, Montgomery makes no mention of extended storage stability as well as the stability of the compositions under exaggerated or accelerated temperature conditions.
U.S. Pat. No. 6,500,408 to Chen details a tooth bleach that utilizes Polyvinylpyrrolidone as a thickening agent in combination with hydrogen peroxide. However, Chen stipulates that the PVP must be present in the formula at a concentration of 25% or greater. He makes no mention of multiple thickening agents within the composition, nor is the PVP thickening agent a cross linked, polyplasdone polymer/hydrogen peroxide complex, contributing synergistically to the available hydrogen peroxide within the total bleach composition. Additionally, Chen makes no mention of long term elevated temperature stability of the tooth bleaching composition.
In contrast, the present invention relates to stable one-part, aqueous tooth whitening composition that utilizes high levels of hydrogen peroxide bleaching agent in a neutral gel formula (pH 6-7). The stability of the composition is proven under accelerated and/or exaggerated storage conditions which translate into at least a two year shelf life for the composition under normal room temperature storage conditions. The stability of the composition is a critical function of known tooth whitening ingredients combined under unique parameters including weight percentages and ratios, for the elevated temperature stability to occur. Deviation outside the controlled composition window, results in predictable failure of the composition.